Carotenoids represent a diverse group of lipid-soluble pigments found in plants, algae, and in many bacteria and fungi, where they serve several purposes including light harvesting, protection against oxidative damage, and attractants to animals and insects for pollination and seed dispersal. Plant carotenoids are synthesized in chloroplasts of photosynthetic tissues, and in chromoplasts of fruits and flowers. In general, carotenoids are C40 terpenoids that consist of eight isoprene units synthesized by the isoprenoid biosynthetic pathway. Significant progress has been made in elucidating the molecular biology of carotenoid biosynthesis, and genes encoding all of the enzymes that are thought to form the backbone of the carotenoids' pathway in plants have been cloned.
In addition to the functions they serve in plants, carotenoids also play an essential role in human nutrition. Beta-carotene is the most well-known carotenoid in the human diet because it is the principal carotenoid used by the body for the synthesis of vitamin A and, as such, is referred to as a provitamin A carotenoid. Vitamin A and its derivatives are important components of nutrition, vision, and cellular differentiation (von Lintig et al., “Filling the Gap in Vitamin A Research,” J. Biol. Chem. 275:11915-11920 (2000)). Lack of vitamin A, especially in children, can lead to blindness (West, C. E., “Meeting Requirements for Vitamin A,” Nutr. Rev. 58:341-345 (2000)). The World Health Organization reports that vitamin A deficiency is a public health problem in 118 countries. They estimate that between 100 and 140 million children are vitamin A deficient and that 250,000 to 500,000 of them become blind every year with one-half of them dying within 12 months of losing their sight. Death rates from common childhood infections such as diarrheal disease and measles increase in children who are vitamin A deficient.
In general, animals are unable to synthesize vitamin A de novo, and, therefore, diet provides the precursors—carotenoids, that are necessary for synthesis of this essential vitamin (von Lintig et al., “Filling the Gap in Vitamin A Research,” J. Biol. Chem. 275:11915-11920 (2000); West, C. E., “Meeting Requirements for Vitamin A,” Nutr. Rev. 58:341-345 (2000)). Sources of vitamin A in the diet include meat and dairy, and there is some fortification of foods. However, vitamin A can also be derived from the provitamin A carotenoids, namely beta-carotene. Though there is, in general, no lack of provitamin A compounds in the Western diet, in some underdeveloped countries the diet is lacking adequate supplies of vitamin A (West, C. E., “Meeting Requirements for Vitamin A,” Nutr. Rev. 58:341-345 (2000)). Recent studies have shown that 21 μg of beta-carotene are required to provide 1 μg of retinol or one retinol equivalent of vitamin A.
In addition, in recent years, there have been reports on the health benefits of other carotenoids, namely lycopene and lutein, the intake of which have been shown to be associated with a decreased risk of various forms of cancer, coronary heart disease, and some degenerative diseases (Krinsky et al., “Biologic Mechanisms of the Protective Role of Lutein and Zeaxanthin in the Eye,” Annual Rev. Nutr. 23:171-201 (2003); Mayne, S. T., “Beta-Carotene, Carotenoids and Disease Prevention in Humans,” FASEB J. 10:690-701 (1996); Osganian et al., “Dietary Carotenoids and Risk of Coronary Artery Disease in Women,” Am. J. Clin. Nutr. 77:1390-1399 (2003); Rock et al., “Update on the Biological Characteristics of the Antioxdant Micronutrients Vitamin C, Vitamin E, and the Carotenoids, J. AM. Diet. Assoc. 96:683-702 (1996)).
The potato (genus Solanum), which originated in the highlands of South America, has been a major food staple for 8,000 years. After wheat, maize, and rice, it is the fourth most important food crop worldwide, and nearly one-third of potato production is in developing countries. In potato tubers, there are two principal classes of pigments. Red, blue, and purple flesh tubers owe their color to anthocyanins, whereas tubers with yellow and orange flesh contain carotenoids (Brown et al., “Orange Flesh Trait in Potato: Inheritance and Carotenoid Content,” J. Amer. Soc. Hort. Sci. 118:145-150 (1993)). The carotenoids that potato accumulates in greatest abundance are xanthophylls. Unlike beta-carotene, xanthophylls cannot be converted by the human body into vitamin A, and, thus, potato is a poor source of this vitamin. Interestingly, in certain potato lines one such xanthophyll that accumulates to high abundance is zeaxanthin, a derivative of beta-carotene that is formed when a hydroxyl group is added to beta-carotene through the activity of β-carotene hydroxylase. The gene encoding beta-carotene hydroxylase has been cloned from plants (Hirshberg, “Molecular Biology of Carotenoid Biosynthesis,” in Britton, eds., Carotenoids, 3, Berlin:Birkhaeuser Verlag, pp. 149-194 (1998)).
The inventors postulated that high zeaxanthin potato lines possess the potential to accumulate large amounts of beta-carotene in their tubers, but do not do so because of the activity of beta-carotene hydroxylase. In theory, reducing beta-carotene hydroxylase activity in such potato tubers should result in the accumulation of beta-carotene because it is the immediate precursor of zeaxanthin. RNA silencing is a means of providing specific and heritable genetic interference through the introduction into a genome of double-stranded RNA-expressing constructs (Chuang et al., “Specific and Heritable Genetic Interference by Double-Stranded RNA in Arabidopsis thaliana,” Proc. Nat'l. Acad. Sci. USA 97:4985-4990 (2000); Waterhouse, et al., “Exploring Plant Genomes by RNA-Induced Gene Silencing,” Nat. Rev. Genet. 4:29-38 (2003)).
The enhancement of beta-carotene accumulation in the potato, by RNA silencing with the activity of β-carotene hydroxylase, provides the potential of increasing vitamin A intake in developing countries, and of providing a source for increased beta-carotene intake in Western diets.
The present invention is directed to overcoming these and other deficiencies in the art.